The present invention relates to a circuit element utilizing spin resonance in a thin magnetic film such as a thin YIG (yttrium iron garnet) film formed on a non-magnetic substrate such as GGG (gadolinium gallium garnet) substrate, and more particularly to the structure of a circuit element utilizing a magnetostatic wave, for suppressing a spurious mode and for enabling the circuit element to operate in a wide frequency range.
It has been proposed to use a thin-film ferrimagnetic resonator having a structure that a thin YIG film is deposited by liquid phase epitaxy on a non-magnetic substrate made of GGG, and then the YIG film is selectively removed so as to have a desired shape, in a microwave oscillation circuit or the like. (refer to a Japanese Patent Application JP-A-2-13101 and others).
This thin-film ferrimagnetic resonator has advantages that the resonance sharpness Q of the resonator is high in the microwave frequency band, and the resonance frequency thereof can be varied depending upon the intensity of a D.C. bias magnetic field applied perpendicularly to the thin ferrimagnetic film coupled magnetically with a transmission line.
Further, a resonator utilizing thin-film ferrimagnetic resonance has been proposed, in which the microwave transmission line is formed on the thin ferrimagnetic film by photolithographic techniques, to adjust the coupling between the thin ferrimagnetic film and the transmission line readily and to enhance the degree of coupling therebetween. (refer to Japanese Patent Application JP-A-62-245704 and others).
As is well known when it is supposed that the volume of a magnetic medium is infinite, the spin resonance occurs abruptly in the magnetic medium when the frequency of an applied microwave becomes equal to a Larmor frequency corresponding to an applied magnetic field. At this time, the phase of spin is constant in the whole of the magnetic medium. That is, the spin in the medium is put in a uniform resonance state. In fact, the volume of a magnetic medium is finite. Accordingly, in order to reduce the magnetostatic energy of the whole of the magnetic medium in accordance with a boundary condition, the phase of spin varies gradually in the magnetic medium. At this time, the change of phase shows the property of a wave. Thus, the change of phase of spin is called "magnetostatic wave".
FIG. 2A shows an example of a conventional circuit element utilizing a magnetostatic wave, and FIG. 2B shows a magnetostatic-wave resonator used in the circuit element of FIG. 2A.
Referring to FIG. 2B, a magnetostatic-wave resonator 6 is formed in such a manner that a thin YIG film 3 is deposited on a GGG substrate 2 by liquid phase epitaxy, a gold or aluminum film is deposited on the YIG film 3 and one or more finger electrodes 5 and a pair of pad electrodes 4a and 4b are formed of a gold or aluminum film deposited on the YIG film 3 through photolithographic techniques so that the pad electrodes 4a and 4b are disposed on both sides of each finger electrode 5. A conventional circuit element 1 shown in FIG. 2A employs the magnetostatic-wave resonator 6 of FIG. 2B. In FIG. 2A, reference numeral 1 designates the circuit element, 6 the magnetostatic-wave resonator, 7 a matching stub, 11 a con-ducts plate, 12a and 12b connecting plates, 13 an conductive plate, 14 a dielectric material, and 15 a microstrip line.
Owing to the mechanism of resonance, the above circuit element using the thin YIG film can operate at temperatures lower than the operation temperation of a conventional circuit element using a YIG sphere. Moreover, the former circuit element of FIG. 2A is relatively inexpensive, because a cumbersome step of forming the YIG sphere is not required.
In a case where, as shown in FIG. 2A, an external magnetic field H.sub.o is applied in a direction perpendicular to the finger electrodes and the thin YIG film, the magnetostatic wave is propregated, as a volume wave, in directions perpendicular to the finger electrodes and parallel to the thin YIG film.
When the band pass characteristics of the circuit element 1 of FIG. 2A are measured, a spurious mode frequently appears in the neighborhood of the lowest one of resonance modes, as shown in FIG. 3A. In many cases, the dependence of the resonance frequency of the spurious mode on the external magnetic field differs a little from the external magnetic field dependence of the resonance frequency of the lowest resonance mode.
Accordingly, as shown in FIG. 3B, the positional relation between the spurious mode and the lowest resonance mode varies with a resonance frequency.
The above fact will be explained below in more detail, with reference to FIGS. 3A and 3B. FIG. 3A shows the spurious mode. Referring to FIG. 3A, a small peak deviating from a main peak indicates the spurious mode. When a resonance spectrum is observed while changing the resonance frequency of the lowest resonance mode by increasing the intensity of the external magnetic field H.sub.o gradually, the spurious mode appearing on one side of the peak due to the lowest resonance mode gradually approaches the peak, passed through the peak, and then moves to the other side of the peak. (refer to LEEE TRANS on MAGNETICS vol. MAG-20, No. 5, Sep. 1984)
Thus, in a case where the above magnetostatic-wave resonator is used for forming, for example, a microwave oscillation circuit, when the operating point of the oscillation circuit on a Smith chart passes through the resonance frequency of the spurious mode as shown in FIG. 4A, the oscillation frequency changes discontinuously as shown in FIG. 5A. That is, there arises a problem that a mode jump of the order of 1 MHz appears. Further, when the spurious mode and the lowest resonance mode overlap each other, the peak due to the lowest resonance mode becomes dull. Thus, there arises another problem that a resonance shapness Q is reduced to a great degree.